1. Field of the Invention
The present invention relates to a mechanism for controlling the position of an optical element in an optical apparatus. More specifically, the present invention relates to a mechanism which moves an optical element holding member via the use of a lead screw and a linearly moving nut (lead nut).
2. Description of the Related Art
In optical apparatuses such as cameras, a mechanism for controlling the position of an optical element with a lead screw and a linearly moving nut is often used. As an example, the following structure is known in the art. Namely, an optical element holding member is guided in a specific direction (e.g., in an optical axis direction or a direction orthogonal to the optical axis direction) by a linear guide device such as a guide shaft, and a lead screw is installed so that the moving direction of the optical element holding member and the rotational axis of the lead screw are parallel to each other. The linearly moving nut, that is screw-engaged with the lead screw, is engaged with the optical element holding member in a state of being prevented from rotating about the rotational axis of the lead screw. The optical element holding member is biased in a direction to remain in contact with the linearly moving nut so as to follow the linearly moving nut in the moving direction of the optical element holding member at all times. Upon rotating the lead screw, the linearly moving nut moves linearly in the direction of the rotational axis of the lead screw, and accordingly the optical element holding member linearly moves with the linearly moving nut. This structure is disclosed in Japanese Unexamined Patent Publication 2000-206391.
The anti-rotation mechanism for preventing the linearly moving nut from rotating is usually composed of a key groove and a projection which is engaged in the key groove. However, if even a slight gap exists in this anti-rotation mechanism, there is a possibility of the linearly moving nut vibrating (rattling) and thereby generating noise, especially when the lead screw is driven intermittently at a high speed. As a countermeasure against such noise, a technique of making the linearly moving nut out of synthetic resin and resiliently deforming the linearly moving nut so that no gap is created in the anti-rotation mechanism. More specifically, in the known art, the linearly moving nut is provided with a resiliently deformable arm that extends along a plane orthogonal to the rotational axis of the lead screw to suppress vibrations of the linearly moving nut by resilient deformation of the arm in the plane. However, to make such resilient deformation for the prevention of vibrations possible, the length of the arm (span from the fixed end of the arm to a force applying portion thereof) needs to be long to a certain degree, which is a restriction on miniaturization of the linearly moving nut. In addition, adopting a vibration suppressing structure using this kind of resiliently deformable arm causes an increase in the distance between the guide shaft, which guides the optical element holding member, and the lead screw. As a consequence, there is a possibility of the linearly moving nut vibrating (chattering) easily when the optical element holding member moves; moreover, there is a possibility of the compact design of the drive mechanism for the optical element holding member deteriorating.